Haven't played in a while, but last I heard this was an issue with the base DF game where the game now keeps track of enemies population, and army movement. Meaning the npcs have to do well enough to actually support sending an army, in addition to its travel time, and whether or not you are a big enough (biggest?) target within range to be worth their time. I had some rounds where i could be 10 years in before i saw the first few kidnapper/thief sightings, which pretty much killed my interest in the game.
Prior to that change the enemy sieges would basically just pop into existence at the edge of your map.
Not sure if all this info is correct, it is just how it was explained to me.